The present invention is directed to the production of drying temperature independent co-polytetrafluoroethylene (PTFE) powder in combination with trace copolymers, effected by charging the reaction vessel with reactants at incremental intervals. The trace copolymers in the PTFE are defined as small amounts of comonomers, less than 1.0 weight percent.
The PTFE art is aware of several variables in need of control to produce consistent high yield products with varying grades of quality, dependent upon the desired PTFE use. Generally, initial reactants are brought together in an autoclave, the time, temperature, and pressure of the reaction being carefully controlled and monitored. At appropriate times certain additional reactants and/or catalysts are added to the mixture to complete the reaction. The autoclave is allowed to cool to room temperature and the "wet" reaction products deposited into drying ovens. In the present invention the fine powder resin was made by polymerizing tetrafluoroethylene (TFE) in an aqueous medium under conditions which maintain the polymer dispersed as fine particles, from 0.05 to 1.3 microns in diameter, until the polymerization reaction is completed. The resultant aqueous dispersion can then be coagulated, dried and subsequently used in this form for extrusion.
An important property of the PTFE fine powder is the force required to extrude a paste through a forming means such as an extrusion die. This force is known as extrusion pressure. The extrusion pressure required to make acceptable extrudate is inversely related to the reduction ratio of the extrusion die. It is known to those skilled in this art that variations in extrusion pressure may be obtained by varying the drying temperature. These variations indicate that the extrusion pressure may therefore be functionally related to drying temperature. This functional relationship is relied on to produce PTFE powder of different grades for many of the useful purposes found for this copolymer. Knowledge of this relationship aids in the control, much like fine tuning, over trimming of the extrusion pressure and therefore control over the effective reduction ratio. This control provides the fine tuning required to produce consistent extrudates.
As a general description of this product reaction, coagulated dispersions or fine powders of PTFE are made by initiating an aqueous dispersion polymerization with free radical peroxide catalysts. The polymerization is kept under constant tetrafluoroethylene (TFE) pressure in a pressurized vessel or autoclave. The resultant PTFE is in latex form and is later coagulated to a fine powder of about 500 microns in diameter by mechanical agitation. The powder is separated from the water in the product mixture with a separation screen. The fine powder is dried, mixed with about 17% by weight naptha or kerosene type lubricant and extruded. The extrusion pressure is determined by: the primary particle size of the PTFE particles in the dispersion, the incorporation of a comonomer in the polymerization, and by the drying temperature.
A major determinant for the extrusion pressure of PTFE fine powder is the primary particle size made in the autoclave. This may be controlled by the concentration of catalyst (number of growth sites) and the concentration of the wetting and/or stabilizing agent in the autoclave during the first 8% of reaction or TFE transfer. For example, for low extrusion pressure fine powder, large primary particle sizes are desired. To achieve the large primary particle sizes, small concentrations of catalyst and small concentrations of wetting agents are desired. The small quantities provide proportionately fewer growth sites, enabling the growth of larger particles. To achieve high extrusion pressure fine powder small primary particle sizes are desired.
By way of example only, low extrusion pressures were obtained when 0.037 weight percent ammonium perfluorooctanate (APFO) were added. To achieve high extrusion pressures with a 0.1 weight percent APFO addition was necessary. In the same manner low extrusion pressures are obtained in the presence of 0.011 weight percent ammonium persulfate (APS) catalyst and high extrusion pressures were obtained in the presence of 0.049 weight percent APS.
Generally, the higher the drying temperature the higher the resultant extrusion pressure. This influence on extrusion pressure is not as great as changing the concentration of the reactants in the autoclave. The drying temperature hones extrusion pressure within the desired range for a particular grade of PTFE. The calculus for this relationship approximates for every 20 degree centigrade increase in drying temperature, the extrusion pressure rises by approximately 10.6%. The converse is true for decreasing drying temperature. This relationship is obtained within the temperature range of greater than 100 degrees centigrade to about 280 degrees centigrade.
Controlling extrusion pressure with drying temperatures has several drawbacks. Firstly, drying time varies inversely with temperature. Therefore, as drying temperatures rise drying times decrease. By way of example, for a drying temperature of 250 degrees centigrade after about 5 hours of warm up time the oven load needs to be at temperature for approximately 8 hours. For a drying temperature of 120 degrees centigrade a warm up time of about 5 hours is needed, however, the oven load requires approximately 24 hours of drying time. This time differential has a large impact on a commercial operation since the lower drying temperature has a major impact on throughput and plant capacity. In this above example the lower drying temperature increases the drying time per batch by more than doubling the time required.
Secondly, to obtain the lowest extrusion pressures drying must be done at the lowest drying temperatures, as for example at the 120 degree centigrade temperature regime. The additional problem at these drying temperatures is that it is close to the vapor phase of water and difficult, therefore, to insure a specified dryness, known as "bone" dry. If bone dryness is not achieved the product will have inconsistent extrusion properties when mixed with the organic extrusion vehicles. The difference in a few degrees centigrade in this regime can be quite dramatic. When dried at 120 degrees centigrade about 6.4 weight percent of the net product will be "wet" or not bone dry. When dried at 150 degrees centigrade about 1 weight percent of the product will be wet. The water contained therein is not homogeneous throughout thereby resulting in inconsistencies of extrusion pressures of the resultant product.
There has been a need in this art for a drying temperature independent or a much less dependent process to produce the desired grades of PTFE. The invention herein disclosed is a discovery of that nature, wherein the resultant PTFE powder extrusion pressure is independent of or exhibits very little dependence on product drying temperature and time. An additional benefit is realized since green strength can be increased while maintaining a substantially constant extrusion pressure. This new process produces a product useful in the same fields of use as any grade of PTFE. Since the new PTFE copolymer extrusion pressure is independent or less dependent on drying temperature the new process is more economical. The uses for the PTFE powder range from tubing and wire coatings, to other useful protective linings such as pipe lining and article wrapping.